In recent years the use of radio frequency ventroposterior pallidotomy (VVP) as a symptomatic treatment for the medically refractory motor symptoms of Parkinson's disease (PD) has been gaining widespread acceptance. A number of centers have reported significant improvements in controlling disabling dyskineasias, on/off fluctuations and other motor symptoms in patients with idiopathic PD a syndrome fluctuations and other motor symptoms in patients with idiopathic PD. Although there is a general agreement that VPP is helpful for many patients, results vary considerably between patients and centers performing the operation. Once reason for this variability is likely due to differences in lesion placement and size. 1. To determine the location and size of pallidotomy lesions in the treatment of medically intractable Parkinson's disease and correlate them with clinical outcome. 2. To determine how Talairach coordinates predict anatomical lesion location. 3. To determine the spatial location of changes in the lesion over time.